The present invention relates to a toner used for developing an electrostatic latent image in an electrophotography, an electrostatic recording, etc.
An imaging process wherein use is made of static electricity, such as electrostatic recording or electrostatic photography, comprises the step of forming an electrostatic latent image and the step of visualizing the electrostatic latent image. The electrostatic latent image is formed by light signal on a photosensitive material prepared by coating a base material such as aluminum and paper with a photoconductive material such as phthalocyanine pigment, selenium, cadmium sulfide and amorphous silicon. The electrostatic latent image thus formed is visualized by subjecting colored fine particles called toner having a particle diameter regulated to 5 to 50 .mu.m to contact chargeability with a charge carrier such as iron powder and ferrite powder (two-component development) or to direct chargeability (one-component development) and then allowing the charged toner to act on the electrostatic latent image. It is necessary to impart a charge corresponding to the polarity of the electrostatic latent image formed on the photoconductive material, that is, either a positive charge or a negative charge, to the toner.
The colored fine particle called toner generally comprises a binder resin as the main component as well as a colorant and charge control agent. Among them, the charge control agent is particularly important component which controls the retainment of the electric charge generated by contact chargeability with a carrier and charging properties of the toner. Although an electric charge can be imparted to the toner through the utilization of a chargeability property of colorant and the binder resin per se without using any charge control agent, only extremely poor image quality can be obtained due to occurrence of fogging phenomena derived from poor chargeability. Quality characteristics required of the toner other than chargeability include excellent stability, fluidity and fixing property. These quality characteristics are greatly affected by the charge control agent used for the toner.
Examples of the conventional charge control agent added to the toner include (1) colored negative charge control agents such as 2:1 metallic complex salt dyes (Japanese Patent Publication (KOKOKU) Nos. 26478/1970 and 20153/1966) and phthalocyanine pigments (Japanese Patent Application Laid-Open (KOKAI) No. 45931/1977), and colorless negative charge control agents such as metal complexes of aromatic dicarboxylic acids (Japanese Patent Publication (KOKOKU) No. 7384/1984), metal complexes of salicylic acid (Japanese Patent Application Laid-Open (KOKAI) No. 104940/1982), or those described in Japanese Patent Application Laid-Open (KOKAI) No. 3149/1986; and (2) positive charge control agents such as nigrosine dyes, triphenylmethane dyes, various quaternary ammonium salts (Bulletin of the Institute of Electrostatics Japan, vol. 4, No. 3, P. 114 (1980)) and organotin compounds such as dibutyltin oxide (Japanese Patent Publication (KOKOKU) No. 29704/1982). The toners containing these compounds as the charge control agent, however, do not sufficiently satisfy the quality characteristics required for the toner, such as chargeability and charge durability (an ability to maintain a charge for a long time).
For example, although the amount of chargeability of the toner containing a 2:1 metallic complex salt dye known as the negative charge control agent is on a fair level, this dye is disadvantageously poor in the dispersibility in a binder resin on the whole. For this reason, the dye is not homogeneously dispersed in the binder resin, and the distribution of charge extremely lacks in sharpness. The resultant image has a low gradation and is poor in the image forming capability.
Further, the 2:1 metallic complex salt dye is disadvantageous because it cannot be used but for a toner having a shade of color limited to black or blackish hue. The use of this dye for a color toner is detrimental to the brightness of the colorant.
Examples of the nearly colorless negative charge control agent include a metal complex of an aromatic dicarboxylic acid (Japanese Patent Publication (KOKOKU) No. 7384/1984). This charge control agent, however, is disadvantageous in that it cannot become completely colorless and the dispersibility is poor. A metal complex of salicylic acid is a colorless charge control agent having relatively good chargeability (Japanese Patent Application Laid-Open (KOKAI) No. 104940/1982), but causes safety problems owing to its inclusion of a heavy metal. Examples of the colorless negative charge control agent containing no heavy metal include compounds disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 3149/1986 and 38958/1988. These compounds, however, are disadvantageous in that it is difficult to produce toners having a good stability because the melting points of the compounds are lower than the processing temperature (180.degree.-260.degree. C.) and various troubles may occur during the toner processing, and that the rise of the charge is slow.
The nigrosine dyes and triphenylmethane dyes known as positive charge control agents as well are colored and therefore can be used only for a toner having a color limited to black or blackish color. They are also poor in the charge durability in continuous copying. The quaternary ammonium salt, when incorporated in a toner, has a poor charge durability attributable to its insufficient moisture resistance and therefore cannot provide an image having a good quality in repeated use.
Japanese Patent Publication (KOKOKU) No. 16109/1992 describes that bisphenol compounds act as good charge control agents in toner. However, these compounds do not exhibit sufficient effect as charge control agents owing to their relatively large particle size, i.e., 10-15 .mu.m.
Thus, the conventional charge control agents do not sufficiently satisfy the quality characteristics requirements for the toner.